In recent years a number of taut-wire intrusion detection systems of varying degrees of sophistication have been developed. Prior known wire fences typically include a detection zone consisting of two anchor posts to which a plurality of trip wires are secured and a centrally located sensor post. Such an arrangement presents the risk that an intruder using the taut wires as footholds very close to the point of attachment of wires to an anchor post may successfully climb such a fence without triggering an alarm, since the deflection of the wires from their normal position in the vicinity of the remote sensor post is then at a minimum compred with an intrusion at other locations along the fence. Increasing the sensitivity of the detectors in the sensor post in such a fence arrangement may lead to frequent false alarms.
Attempts have been made to lessen the risk of an intrusion in the "dead" vicinity of an anchor post by providing wire-anchoring members at regular intervals along the fence which are of a geometry or configuration that is more difficult to scale than a vertical anchor post. A ground-anchoring means comprising a diagonally secured post to which the trip wires are secured is illustrated in U.S. Pat. No. 4,533,906 (Amir). This and other arrangements, however, do not address the fundamental problem of the presence in a security fence of a plurality of wire-anchoring regions which are necessarily less sensitive to intrusion than regions remote from the anchoring of the wires, where an intrusion attempt will cause more substantial wire deflections.
A further disadvantage presented by prior fence alarm systems is their inability to distinguish between the numerous different kinds of intrusion events which can be presented and to distinguish these from each other and from non-intrusive events (false alarms) caused by changing environmental conditions, and the like. In particular, prior known taut-wire fence alarm systems have an arrangemenet of sensors on detector posts such that a detector post will go into an alarm condition once any or all of the trip-wires associated with the detector post is deflected by some pre-determined threshold amount. A greater sophistication of signal analysis is required to distinguish, for example, between attempts to penetrate the taut-wire barrier by climbing, an attempt to pass between the wires by spreading two of them apart, "false" signals caused by severe weather conditions, and so on.